Cloud Brightening - a Clever Way to Brighten our Future?Cloud Brightening - a Clever Way to Brighten our Future? Sabine Undorf PhD Student Global Change Research Institute School of

Cloud Brightening - a Clever Way to Brighten our Future?

Sabine Undorf

PhD StudentGlobal Change Research Institute

School of GeoSciencesUniversity of Edinburgh

February 2015

Anthropogenic climate change is the biggest challenge humankind has encountered so far, given its worldwide

impacts, the strength and variety of these impacts, its persistence into the future as well as the difficulty to

mitigate it [IPCC14a]. Since the dominant cause of global warming is the increased levels of greenhouse gases

(GHGs) in the atmosphere [IPCC13], the direct way to limit further warming is to stop emitting greenhouse

gases, mainly carbon dioxide (CO2) from fossil fuel combustion and industrial processes [IPCC14b]. Although

the United Nations agreed on the objective of “[stabilising] greenhouse gas concentrations in the atmosphere

at a level that would prevent dangerous anthropogenic interference with the climate system” [UN92], progress

in reducing greenhouse gas emissions has been slow – it requires huge international political, economical and

technical efforts, which makes it a difficult and lengthy process. Additionally, if this changed abruptly in the

near future, global mean temperature would only be affected on a long time-scale because of the inertia in

the carbon cycle [vVS13]. Methods to intentionally alter the climate system in order to reduce climate change

(referred to as geoengineering or climate engineering, [Kei00]) have thus been suggested to complement mit-

igation strategies [Roy09], since they might be able to counteract some manifestations of climate change on

a smaller time scale. One of these ideas is to brighten marine clouds in order to induce a cooling effect. How

would this work and is it a good idea? In order to answer these questions, we need to understand something

about the role of clouds in the climate system, the concept of radiative forcing, and the effect of aerosols in

the atmosphere.

We can define climate as the synthesis of weather (temperature, precipitation, etc.) in a particular re-

gion over time [Har94]. Surface temperature is the result of an energy balance between (shortwave) incoming

solar radiation and outgoing (longwave) radiation from the Earth, while the albedo (reflectivity) of the surface

as well as the wavelength dependent radiative properties of the atmosphere affect this balance. The atmo-

sphere consists primarily of gases, but also contains liquid and solid particles, specifically liquid or solid water,

for instance as cloud particles, and aerosol particles [BRA+13], and they interact with each other and with

radiation. Any change in the composition of the atmosphere and other climate system components (and land

use) changes the radiative balance, i.e. it imposes a radiative forcing (RF). The climate system reacts with

complex adjustments and feedback mechanisms, and in addition to a different surface temperature, changes

in large-scale circulation patterns, precipitation etc. will occur. While GHGs like water vapour and CO2

reduce the atmosphere’s transmissivity for outgoing longwave radiation, and thus apply a positive RF (the

well-known greenhouse effect), there is ”high confidence that aerosols and their interactions with clouds have

offset a substantial portion of global mean forcing from well-mixed greenhouse gases” [IPCC13]. This essay

will now move on to outline the processes through which this takes place.

1

Sabine Undorf – Cloud Brightening IOP Environmental Physics Essay Competition 2015

(a) low-level cloud in clean air (b) brighter low-level cloud in polluted air

Figure 1: Aerosol-cloud interactions and their impact on climate. The situation in clean air, i.e. in thepresence of few aerosols, is shown in (a) compared to that in polluted air shown in (b). Aerosols serve asCCN, and more aerosols result in a higher concentration of smaller droplets. This leads to brighter clouds, asindicated by the thicker arrow representing outgoing radiation in (b) compared to (a). From [BRA+13].

Clouds impose both negative and positive RF components – the former by enhancing the Earth’s albedo, and

the latter by contributing to the greenhouse effect. Satellite measurements are used to estimate the RF of the

two effects as about -50 Wm−2 and +30 Wm−2, respectively, which add up to an overall cooling effect from

clouds in the current climate [BRA+13]. The relative size of the warming and cooling components depend on

the type of cloud. One characteristic is height, since the temperature of the atmosphere is height dependent.

High clouds can be colder than the clear-sky radiating temperature, thus the cloud-covered sky emits less

long-wave radiation and has a warming effect. Another characteristic is the optical thickness of clouds: the

higher the optical thickness, the less sunlight is transmitted – optical thick clouds thus induce a cooling effect

on the surface. So how could the number of overall cooling clouds be increased, or how could they be made

even more cooling? This is where aerosols come into play.

Atmospheric aerosols originate from both natural sources and anthropogenic sources. The former include

mainly sea salt and dust in the troposphere, as well as sulphur aerosols from volcanic eruptions in the strato-

sphere, while the latter include sulphate and carbonaceous aerosols and arise primarily from combustion

processes [CCSP09]. Aerosols exert an RF by themselves, which is the direct effect of scattering and absorb-

ing sunlight. Both scattering aerosols (e.g. sulphate) and absorbing aerosols (e.g. black carbon) exist, which

cool and warm, respectively. But aerosols also interact with clouds by serving as condensation and ice nuclei,

which facilitates cloud formation. Changes in aerosol density thus lead to indirect effects: the so-called cloud

albedo effect, [Two77], and effects from clouds adjusting rapidly to changes in their environment (lifetime

effect, [Alb89] and others). The cloud albedo effect is illustrated in Figure 1: If more aerosols are available as

cloud condensation nuclei (CCN), more cloud droplets will form, but they will be smaller in size if the liquid

water content is the same. This increases the total droplet surface area and thereby the albedo [BRA+13].

The cloud albedo effect is visible in ship tracks, which form around the exhaust released by ships.1 Figure 2

presents satellite images of ship tracks, which show the clouds as we see them, their optical thickness and ef-

fective cloud particle radius. The albedo effect is illustrated by the resemblance of the images: where aerosols

are emitted along the ship travel routes, the effective particle radius is smaller, the clouds are optically thicker

and look brighter.

1Contrails from aircraft are another example for cloud formation as a consequence of the release of anthropogenic aerosolsinto the atmosphere. Their climate impact, however, is different, since planes are much higher in the atmosphere than ships[Gie06].

2


Figure 2: Ship tracks in the Atlantic observed with the Moderate-Resolution Imaging Spectroradiometer(MODIS) on the Aqua satellite on January 27, 2003. The true colour image is shown (left) as well as theretrieved optical thickness (middle) and the effective particle radius (right) for a section of the true-colourpicture marked with a black frame. Modified from [DG03].

The idea is thus clear: utilise this effect by increasing aerosol density on a larger scale so that a significant

cooling effect is created, and we would all need to worry a bit less about anthropogenic global warming! How

would we choose where to do it, and how would we do it? We would seek regions with (1) the presence of

suitable clouds, (2) naturally clean air, (3) plenty of sunshine to reflect, and (4) low impact on the regional

climate of inhabited areas (and a few more criteria, partly specific to the technique used to apply the ad-

ditional aerosols [SSL08]). The second criterion puts oceans in the focus, since due to their short lifetime

in the atmosphere, aerosols tend to stay in the regions where they are emitted, and over open sea, there

are few emission sources. There CCN consist primarily of sea salt aerosols originating from sea spray, and

oxidising products of an organic sulphur compound called dimethylsulphide (DMS) [YB02], which is mainly

produced by phytoplankton [SST+07]. It is generally agreed that marine stratocumulus cloud with relatively

weak precipitation are optimal candidates to be brightened due to “their relatively low droplet concentrations,

their expected increase in cloud water in response to seeding, and the longer lifetime of sea salt particles

in non- or weakly precipitating environments” [BRA+13]. Therefore, these clouds might be brightened by

directly injected sea-spray particles into the air [Lat90]. Ideas to realise this include wind-driven rotor boats

with turbines generating electrical energy to create sea spray [SSL08]. Cost estimates for research and devel-

opment, setting-up and maintenance of such a fleet are very uncertain, most often total annual costs in the

range of a billion seem to be assumed ([LRC+08], [Roy09]). The indirect increase of the aerosol density by

enhancing the production of DMS [CLAW87] through iron fertilisation has also been suggested [WEB07], but

its efficacy is disputed [VVvG08, WMCB08, WEB08].

Figure 3 is an exemplary demonstration of the change in RF for an increased CCN density in regions of

low-level maritime cloud, as simulated by a global circulation model (GCM). The potential RF imposed by

modifying cloud brightness is the subject of numerous studies, and the value of the assessed RF as a function

of ocean surface coverage varies significantly. Most results using GCM simulations indicate that a stabilisation

of the Earth’s temperature about its current level would be feasible for some decades provided that the models

represent aerosols and their interactions with clouds acceptably well [LBC+12]. These effects, however, “con-

tinue to contribute the largest uncertainty to the total RF estimate” [IPCC13]. The IPCC thus assesses that

“evidence that cloud brightening methods are effective and feasible in changing cloud reflectivity is ambiguous

and subject to many of the uncertainties associated with aerosol–cloud interactions more broadly” [BRA+13].

3


Figure 3: Simulated change in RF (Wm−2) at the top of the atmosphere for an increasing density of CCN, asit could be achieved by injecting aerosols. The five-year mean difference between a control simulation (withCCN of 100 cm−3) and a test run with increased CCN of 375 cm−3 in regions of low-level maritime cloud isshown. From [Roy09], an extension of results from [LRC+08].

In addition to insufficient knowledge on the efficacy of cloud brightening, there are further concerns, related

to (1) risks associated with geoengineering in general, (2) its subgroup of solar radiation management (SRM)

methods, and (3) this proposed method in particular, leaving unsolved technical challenges aside. A risk of the

third kind is that cloud brightening would introduce a negative RF locally, and large local changes in RF can

result in larger-scale oceanic and atmospheric circulation changes. The impact of spatially non-homogeneous

forcing by anthropogenic aerosols (as opposed to homogeneous GHG forcing) has been detected in a multitude

of aspects of the twentieth-century climate. An example is the summertime drying of the South Asian mon-

soon ([BMR11] and others), which impacts a large share of Earth’s population. If cloud brightening turned

out to have similar side effects, it would obviously not work well as a method to alter the climate system in

order to prevent negative consequences of anthropogenic global warming.

SRM methods all aim at reducing the amount of sunlight reaching the Earth surface; besides cloud brightening,

the injection of stratospheric aerosols in order to mimic the cooling effect observed after large volcanic erup-

tions is another widely discussed method. Generally, the cooling from SRM could never perfectly compensate

for GHG warming, for example because it only changes the heating rates during daytime [BRA+13]. SRM

would have to be replenished constantly as long as increased GHG forcing persists – without CO2 removal

efforts, this will be many centuries to more than thousand years [IPCC13]. A sudden stop in its implemen-

tation is thus not unlikely, for example due to changing political decisions, and poses another risk. Figure

4 shows the response of global temperature and precipitation to such a sudden stop in SRM, as simulated

by GCMs: a rapid return to levels consistent with the CO2 forcing. The increased rate of this warming is

expected to have even stronger effects on ecosystems and human adaption than warming without SRM in the

first place [OCW+14], and could lead to positive feedback further amplifying the warming [MC07]. Other

concerns are non-climatic in nature, such as the risk of international conflict -countries or non-state actors

might be able to implement this method unilaterally due to its relatively low direct costs- or the risk of a moral

hazard, i.e. reduced efforts to mitigate and adopt climate change when relying on SRM. The latter would

4


(a) (b)

Figure 4: [BRA+13] Simulated globally averaged surface temperature (◦C) (a) and precipitation (mm/day)(b) in a scenario of 1 %/yr increase of GHG, while temporarily balanced by SRM application (solid lines) andwithout (dashed lines). The application of SRM is assumed to stop after 30 years. Modified from [BRA+13]([KRB+11]).

be particularly risky, since SRM does not tackle other, non-temperature impacts of increased GHG levels,

like ocean acidification and the influence on plants’ photosynthesis and water management. The complex

questions of justice between people in different regions on Earth as well as present and future generations

raised by anthropogenic climate change would get more complex as well. The IPCC concludes that “SRM

is a potential key risk because it is associated with impacts to society and ecosystems that could be large in

magnitude and widespread“ [OCW+14].

The concept of counteracting anthropogenic global warming by cloud brightening -based on the concep-

tually simple idea of exploiting the positive effect of aerosol number density on cloud particle number and

thereby on clouds’ albedo- would thus come with a variety of risks and concerns, both of environmental and

sociopolitical nature, if it was to be realised on a scale large enough to impose a significant cooling. However,

it seems to be a comparatively quick, cheap and -possibly- effective method [Roy09, KR12] to reduce the

amount of solar energy reaching the Earth surface and thus to counteract some of the climatic effects of our

historical and present (and potentially future) GHG emissions. Due to its imperfect compensation of GHG

warming, cloud brightening and other SRM methods are not suited to replace mitigation and adaption efforts,

and more research needs to be done on the topic before its application as a short-term complementation to

these efforts should be seriously considered. A final decision then needs to be the result of carefully trading

the presented risks off the risks and -potentially irreversible- consequences of conservatively mitigated climate

change, maybe supported by CO2 removal techniques.

5


References

[Alb89] Albrecht, B. A. (1989). Aerosols, Cloud Microphysics, and Fractional Cloudiness. Science, 245:1227–1230.

[BMR11] Bollasina, M. A., Ming, Y., and Ramaswamy, V. (2011). Anthropogenic Aerosols and the Weakeningof the South Asian Summer Monsoon. Science, 334:502–505.

[BRA+13] Boucher, O., Randall, D., Artaxo, P., Bretherton, C., Feingold, G., Forster, P., Kerminen, V.-M.,Kondo, Y., Liao, H., Lohmann, U., Rasch, P., Satheesh, S. K., Sherwood, S., Stevens, B., and Zhang,X. Y. (2013). Clouds and Aerosols. In Stockers, T. F., Qin, D., Plattner, G.-K., Tignor, M., Allen, S. K.,Boschung, J., Nauels, A., Xia, Y., Bex, V., and Midgley, P. M., editors, Climate Change 2013: The PhysicalScience Basis. Contribution of Working Group I to the Fifth Assessment Report of the IntergovernmentalPanel on Climate Change, pages 571–657. Cambridge University Press, Cambridge, United Kingdom andNew York, NY, USA.

[CCSP09] CCSP (2009). Atmospheric Aerosol Properties and Climate Impacts. In Chin, M., Kahn, R. A., andSchwartz, S. E., editors, A Report by the U.S. Climate Change Science Program and the Subcommittee onGlobal Change Research. National Aeronautics and Space Administration, Washington, D. C., USA.

[CLAW87] Charlson, R. J., Lovelock, J. E., Andreae, O. M., and Warren, S. G. (1987). Oceanic Phytoplank-ton, Atmospheric Sulphur, Cloud Albedo and Climate. Nature, 326:655–661.

[DG03] Descloitres, J. and Gray, M. (2003). Ship Tracks off Europe’s Atlantic Coast. Mod-erate Resolution Imaging Spectroradiometer (MODIS), NASA Goddard Space Flight Center,http://earthobservatory.nasa.gov/IOTD/view.php?id=3275&eocn=image&eoci=related image, retrieved2/2015.

[Gie06] Gierens, K. (2006). Contrails, Contrail Cirrus, and Ship Tracks. In Proc. Transp. Atmos. Clim. Conf.,pages 214–220, Oxford, UK.

[Har94] Hartmann, D. L. (1994). Global Physical Climatology. Academic Press, London, UK and San Diego,California, USA.

[IPCC13] IPCC (2013). Summary for Policymakers. In Stocker, T. F., Qin, D., Plattner, G.-K., Tignor, M.,Allen, S. K., Boschung, J., Nauels, A., Xia, Y., Bex, V., and Midgley, P. M., editors, Climate Change2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report ofthe Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdomand New York, NY, USA.

[IPCC14a] IPCC (2014a). Summary for Policymakers. In Climate Change 2014 Synthesis Report.

[IPCC14b] IPCC (2014b). Summary for Policymakers. In Edenhofer, O., Pichs-Madruga, R., Sokona, Y.,Farahani, E., Kadner, S., Seyboth, K., Adler, A., Baum, I., Brunner, S., Eickemeier, P., Kriemann, B.,Savolainen, J., Schlomer, S., von Stechow, C., Zwickel, T., and Minx, J., editors, Climate Change 2014:Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of theIntergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom andNew York, NY, USA.

[Kei00] Keith, D. W. (2000). Geoengineering the Climate: History and Prospect. Annu. Rev. Energ. Environ.,25:245–84.

[KR12] Klepper, G. and Rickels, W. (2012). The Real Economics of Climate Engineering. Econ. Res. Int.,2012:1–20.

[KRB+11] Krawitz, B., Robock, A., Boucher, O., Schmidt, H., Taylor, K. E., Stenchikov, G., and Schulz, M.(2011). The Geoengineering Model Intercomparison Project (GeoMIP). Atmos. Sci. Lett., 12:162–167.

[Lat90] Latham, J. (1990). Control of Global Warming? Nature, 347(6291):339–340.

6


[LBC+12] Latham, J., Bower, K., Choularton, T., Coe, H., Connolly, P., Cooper, G., Craft, T., Foster, J.,Gadian, A., Galbraith, L., Iacovides, H., Johnston, D., Launder, B., Leslie, B., Meyer, J., Neukermans, A.,Ormond, B., Parkes, B., Rasch, P., Rush, J., Salter, S., Stevenson, T., Wang, H., Wang, Q., and Wood,R. (2012). Marine Cloud Brightening. Philos. Trans. R. Soc., 370:4217–4262.

[LRC+08] Latham, J., Rasch, P., Chen, C.-C. J., Kettles, L., Gadian, A., Gettelman, A., Morrison, H., Bower,K., and Choularton, T. (2008). Global Temperature Stabilization via Controlled Albedo Enhancement ofLow-Level Maritime Clouds. Philos. Trans. R. Soc., 366:3969–3987.

[MC07] Matthews, H. D. and Caldeira, K. (2007). Transient Climate–Carbon Simulations of PlanetaryGeoengineering. In PNAS, volume 104, pages 9949–9954.

[OCW+14] Oppenheimer, M., Campos, M., Warren, R., Birkmann, J., Luber, G., O’Neill, B., and Takahashi,K. (2014). Emergent Risks and Key Vulnerabilities. In Field, C. B., Barros, V. R., Dokken, D. J., Mach,K. J., Mastrandrea, M. D., Bilir, T. E., Chatterjee, M., Ebi, K. L., Estrada, Y. O., Genova, R. C., Girma,B., Kissel, E. S., Levy, A. N., MacCracken, S., Mastrandrea, P. R., and White, L. L., editors, ClimateChange 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects. Contributionof Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change,pages 1039–1099. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

[SSL08] Salter, S., Sortino, G., and Latham, J. (2008). Sea-Going Hardware for the Cloud Albedo Methodof Reversing Global Warming. Philos. Trans. R. Soc., 366(August):3989–4006.

[SST+07] Stefels, J., Steinke, M., Turner, S., Malin, G., and Belviso, S. (2007). Environmental Constraintson the Production and Removal of the Climatically Active Gas Dimethylsulphide (DMS) and Implicationsfor Ecosystem Modelling. Biogeochemistry, 83:245–275.

[Roy09] The Royal Society (2009). Geoengineering the Climate. Science, Governance and Uncertainty. RoyalSociety, London, UK.

[Two77] Twomey, S. (1977). The Influence of Pollution on the Shortwave Albedo of Clouds. J. Atmos. Sci.,34:1149–1152.

[UN92] United Nations (1992). United Nations Framework Convention on Climate Change. New York.

[vVS13] van Vuuren, D. P. and Stehfest, E. (2013). If Climate Action Becomes Urgent: The Importance ofResponse Times for Various Climate Strategies. Clim. Chang., 121:473–486.

[VVvG08] Vogt, M., Vallina, S., and von Glasow, R. (2008). New Directions: Correspondence on “Enhancingthe Natural Cycle to Slow Global Warming”. Atmos. Environ., 42:4803–4809.

[WEB07] Wingenter, O. W., Elliot, S. M., and Blake, D. R. (2007). New Directions: Enhancing the NaturalSulfur Cycle to Slow Global Warming. Atmos. Environ., 41:7373–7375.

[WEB08] Wingenter, O. W., Elliot, S. M., and Blake, D. R. (2008). Authors Response to the above Commentby M. Vogt et al. on “New Directions: Enhancing the Natural Cycle to Slow Global Warming”. Atmos.Environ., 42:4806–4809.

[WMCB08] Woodhouse, M. T., Mann, G. W., Carslaw, K. S., and Boucher, O. (2008). New Directions :The Impact of Oceanic Iron Fertilisation on Cloud Condensation Nuclei. Atmos. Environ., 42:5728–5730.

[YB02] Yoon, Y. J. and Brimblecombe, P. (2002). Modelling the Contribution of Sea Salt and DimethylSulfide Derived Aerosol to Marine CCN. Atmos. Chem. Phys., 2:17–30.

7

Documents

Cloud Brightening - a Clever Way to Brighten our Future?Cloud Brightening - a Clever Way to Brighten our Future? Sabine Undorf PhD Student Global Change Research Institute School of